Many point defects have been studied in synthetic diamond material including: silicon containing defects such as silicon-vacancy defects (Si-V), silicon di-vacancy defects (Si-V2), silicon-vacancy-hydrogen defects (Si-V:H), silicon di-vacancy hydrogen defects (S-V2:H); nickel containing defect; chromium containing defects; and nitrogen containing defects such as nitrogen-vacancy defects (N-V), di-nitrogen vacancy defects (N-V-N), and nitrogen-vacancy-hydrogen defects (N-V-H). These defects are typically found in a neutral charge state or in a negative charge state.
Fluorescent point defects in synthetic diamond material have been proposed for use in various sensing, detecting, and quantum processing applications including: magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; spin resonance imaging devices for magnetic resonance imaging (MRI); and quantum information processing devices such as for quantum computing.
In addition to the above, it has also been proposed to use fluorescent point defects in diamond material as fluorescent markers or labels in biological applications and medical diagnostics. For example, Rabeau et al, (Nano Letters, vol. 7, No. 11, 3433-3437, 2007) disclose the use of nanodiamonds as fluorescent labels in biological systems. As indicated by Rabeau et al, key advantages of nanodiamonds compared to other conventional fluorescent biolabels include their noncytotoxicity, room-temperature photostability, and the relative ease with which surfaces can be functionalized. It is further indicated that biological applications demand bright fluorescence from small crystals. In this regard, Rabeau et al. have performed an analysis of diamond particle size versus nitrogen-vacancy (NV) centre content and found a strong dependence of NV centre content and crystal size for diamond nano-crystals grown via a chemical vapour deposition technique. They report that a particle size of 60-70 nm is optimal for single NV centre incorporation per diamond nano-particle.
One problem with the diamond nano-particles described by Rabeau is that they have a low NV centre content and thus have a relatively low fluorescent intensity which is not ideal for many fluorescent marker applications. The Rabeau et al. document itself indicates that biological applications demand bright fluorescence from small crystals. However, there is no indication of how to incorporate a high concentration of NV centres into small diamond nano-crystals to increase their fluorescent intensity. The diamond nano-crystals described in the Rabeau et al, document have a low NV centre content and thus will have a relatively low fluorescent intensity not suited to many fluorescent marker applications.
US2014/0065424 (granted as U.S. Pat. No. 8,932,554) also discloses a method of producing light-emitting nano-particles of diamond. In the method described in this document micrometre scale diamond particles are irradiated and annealed and then the particles are ground to nano-particles having a size between 15 and 20 nanometres.
It is an aim of embodiments of the present invention to provide small diamond particles which have a high concentration of fluorescent point defects and thus a high fluorescent intensity. It is also an aim of embodiments of the present invention to provide suitable fabrication methods for achieving such highly fluorescent diamond particles.